Aluminum-silicon alloys having improved mechanical properties

ABSTRACT

A thermal treatment process for an article of a cast or wrought aluminum-silicon alloy with an eutectic phase. The process comprises a rapid heating of the article to an annealing temperature of 400° C. to 555° C. and maintaining the article at this temperature for a not more than 14.8 minutes.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 10/837,665, filed May 4, 2004, which is a continuation ofInternational Application No. PCT/AT02/00309, filed Nov. 5, 2002, whichclaims priority under 35 U.S.C. §119 of Austrian Patent Application A1733/2001, filed Nov. 5, 2001. The entire disclosures of the parent andgrandparent applications are expressly incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for improving the mechanicalproperties of aluminum-silicon alloys. More specifically, the presentinvention relates to a thermal treatment process for improving theductility of articles of a preferably refined or purified cast orwrought aluminum-silicon alloy with an eutectic phase, which optionallycontains other alloying and/or contaminating elements, said articlesbeing subjected to an annealing treatment and subsequent aging.

Further, the present invention relates to an aluminum-silicon alloy thatcontains at least one processing element, optionally magnesium, as wellas additional alloying and/or contaminating elements with an eutecticphase consisting essentially of an α_(Al)-matrix and siliconprecipitates.

2. Discussion of Background Information

With silicon, aluminum forms a simple eutectic system, the eutecticpoint being at a silicon concentration of 12.5%-wt and a temperature of577° C.

By alloying magnesium, which can be dissolved in the α_(Al)-matrix up toa content of at most 0.47%-wt at a temperature of about 550° C., it ispossible to achieve a considerable increase in the strength of thematerial by means of thermal treatment and the Mg₂Si precipitates formedthereby.

When an Al—Si—Mg smelt cools, the residual smelt can hardeneutectically, whereby silicon separates out therein in a coarse,lamellar form. For a considerable time sodium or strontium have beenadded to alloys of this kind to thereby impede the growth of the siliconcrystals during hardening; this is referred to as enrichment orrefinement and always results in an improvement of the mechanicalproperties, in particular an improvement of elongation at fracture.

The mechanical properties of semifinished products or of articles ofaluminum alloys can be greatly influenced by thermal treatment methods,and the thermal treatment states are defined in European Standard EN515. In this Standard, the letter F stands for “production state” and Tstands for “thermally treated to stable states.” The particular thermaltreatment state is characterized by the number that is associated withthe letter T.

In the following description the thermal treatment states of thematerial are indicated by the following short forms:

-   F—production state-   T5—quenched from the production temperature and thermally aged-   T6—solution quenched and thermally aged-   T6x—thermally treated according to the present invention-   T4x—thermally treated according to the present invention

On the one hand, the properties of the material and, on the other hand,the costs or economic factors involved in production are important formarketing or the industrial use of objects of Al—Si alloys, since inparticular long annealing treatments at high temperatures and thestraightening processes that may be necessitated by so-calledgravitational creep during protracted annealing are themselves costly.

In principle, it can be said that an Al—Si alloy in State F has for themost part a low material strength R_(p) and a relatively high value ofthe elongation at fracture A.

At a thermal treatment state T5, which is to say quenched from theproduction temperature and thermally aged, for example at 155° C. to190° C. for a period of 1 to 12 hours, higher strength values R_(p) willbe achieved, but at lower elongation at fracture values A of thesamples.

At a thermal-treatment state corresponding to T6, with solutionannealing at a temperature of, for example, 540° C. for a period of 12hours and subsequent thermal aging, it is possible to achieve asignificant increase in the strength of the material at an almostidentical elongation at fracture of the samples, or ductility of thematerial as compared to State F. The long duration of the solutionannealing permits an advantageous diffusion of the magnesium atoms inthe material, for example, whereby after quenching and thermal aging ofthe article fine, evenly distributed Mg₂Si precipitates are formed inthe a_(m)-matrix, and these precipitates result in a significantincrease in the strength of the material.

As discussed above, solution annealing at high temperatures and forprotracted periods entails the disadvantages of gravitational creep andcostly temperature-time treatment schedule. For reasons of economy, veryfrequently achieving great strength and good ductility of the materialby T6 is abandoned and a treatment state T5 is selected for the article.The lower strength that results from T5 must, if necessary, becompensated for by making design changes to the component in question.

It would be desirable to provide a new, cost-effective method of thermaltreatment, with which the ductility of the material can be greatlyincreased without causing major losses in material strength as comparedto T6, or with which significantly greater ductility and greatermaterial strength can be achieved in comparison to T5.

It would also be desirable to have available a microstructure of anarticle of the type described in the introduction hereof, which resultsin advantageous mechanical properties of the material.

SUMMARY OF THE INVENTION

The present invention provides a thermal treatment process for improvingthe material ductility of an article which comprises a cast or wroughtaluminum-silicon alloy with an eutectic phase. This process comprisessubjecting the article to an annealing treatment and a subsequent agingtreatment. The annealing treatment is carried out as a shock annealingtreatment which comprises (a) a rapid heating of the material to anannealing temperature of 400° C. to 555° C., (b) maintaining thematerial at this temperature for a holding period of not more than 14.8minutes, and (c) a subsequent forced cooling of the material toessentially room temperature.

In one aspect of the process, the aluminum-silicon alloy may furthercomprise one or more alloying elements and/or one or more contaminatingelements. For example, the aluminum-silicon alloy may further comprisesMg, Mn and/or Fe.

In another aspect of the process, the aluminum-silicon alloy may berefined and/or purified.

In another aspect of the process, the holding period may be shorter than6.8 minutes and/or the holding period may be not shorter than 1.7minutes. For example, the holding period may be not longer than 5minutes.

In a still further aspect of the process, the aging treatment maycomprise a treatment at a temperature of from 150° C. to 200° C., e.g.,for from 1 to 14 hours.

In yet another aspect of the process, the aging treatment may comprise acold aging treatment at essentially room temperature.

In another aspect of the process, the holding period may be from 1.7 toless than 6.8 minutes and the aging treatment may comprise a treatmentat a temperature of from 150° C. to 200° for from 1 to 14 hours, or acold aging treatment at essentially room temperature.

In yet another aspect, the article may have been made by a thixocastingmethod.

The present invention also provides an article which is obtainable bythe above process, including the various aspects thereof.

The present invention also provides an article which comprises analuminum-silicon alloy with an eutectic phase. The eutectic phaseconsists essentially of an α_(Al)-matrix and spheroidized siliconprecipitates. These precipitates have an average cross-sectional area,A_(Si), of not more than 4 μm², A_(Si) being represented by thefollowing equation:

$A_{Si} = {{\frac{1}{n}{\sum\limits_{k = 1}^{n}A}} \leq {4\mspace{11mu} {µm}^{2}}}$

wherein

-   -   A_(Si)=average area of the silicon particles in μm²    -   A=average area of the silicon particles per image, in μm²    -   n=number of images sampled.

In one aspect of the article, A_(Si) may be less than 2 μm².

In another aspect of the article, the aluminum-silicon alloy may furthercomprise one or more alloying elements and/or one or more contaminatingelements. For example, the alloy may further comprises Mg, Mn and/or Fe.

In another aspect, the aluminum-silicon alloy may further comprise atleast one processing element.

The present invention further provides an article which comprises analuminum-silicon alloy with an eutectic phase. The eutectic phaseconsists essentially of an α_(Al)-matrix and silicon precipitatescomprising silicon particles. The average free path length between thesilicon particles, λ_(Si), in the eutectic phase is not higher than 4μm. λ_(Si) is represented by the following equation:

$\lambda_{Si} = {{\frac{1}{n}{\sum\limits_{k = 1}^{n}\sqrt{\frac{A_{square}}{N_{Silicon}}}}} \leq {4\mspace{11mu} {µm}}}$

wherein

-   -   λ_(Si)=average spacing between the silicon particles    -   A_(square)=square reference area, in μm²    -   N_(Silicon)=number of silicon particles    -   n=number of images sampled.

In one aspect of this article, the average free path length may be lessthan 3 μm, e.g., less than 2 μm.

In another aspect of the article, the aluminum-silicon alloy may furthercomprise one or more alloying elements and/or one or more contaminatingelements. For example, the alloy may further comprises Mg, Mn and/or Fe.

In another aspect, the aluminum-silicon alloy may further comprise atleast one processing element.

The present invention further provides an article which comprises analuminum-silicon alloy with an eutectic phase. The eutectic phaseconsists essentially of an α_(Al)-matrix and silicon precipitates whichcomprise silicon particles. The average spheroidization density, ζ_(Si),thereof, defined as the number of silicon particles per 100 μm², is atleast 10:

$\xi_{Si} = {{\frac{1}{n}{\sum\limits_{k = 1}^{n}{\frac{N_{Si}}{A} \times 100}}} \geq 10}$

wherein

-   -   ζ_(Si)=average spheroidization density of the eutectic Si        particles    -   N_(Si)=number of silicon particles    -   A=reference area in μm²    -   n=number of images sampled.

In one aspect of the article, the average spheroidization density may begreater than 20.

In another aspect of the article, the aluminum-silicon alloy may furthercomprise one or more alloying elements and/or one or more contaminatingelements. For example, the alloy may further comprises Mg, Mn and/or Fe.

In another aspect, the aluminum-silicon alloy may further comprise atleast one processing element.

The present invention also provides any of the above articles, includingthe various aspects thereof, which is made by a thixocasting method andis heat treated by a process according to the present invention as setforth above, including the various aspects thereof.

According to the present invention, the solution annealing is conductedas shock annealing which comprises rapid heating of the material to anannealing temperature of 400° C. to 555° C., maintaining it at thistemperature for a period of at most 14.8 minutes, and subsequent forcedcooling, essentially to room temperature.

The advantages that are obtained are that the highest ductility valuesare achieved for the material by a simple short-time high-temperatureannealing. In addition, the so-called shock annealing causes little orno component deformation or warping of the article, so that there is noneed to straighten it. The short-time annealing treatment is veryeconomical and can be incorporated very easily into a productionsequence, for example by using a continuous heating furnace. Materialstrength can be adjusted by an adapted thermal aging technology. Withthe majority of Al—Si alloys, the greatest increase will be achieved if,as can be provided for, the shock annealing is effected with a holdingtime of less than 6.8 minutes, preferably for a period ranging from 1.7up to optionally at most 5 minutes.

If the article is thermally aged after the shock annealing, it isadvantageous to do this at a temperature in the range between 150° C.and 200° C., for a period ranging from 1 to 14 hours.

It can also be advantageous from the material standpoint if the aging ofthe article that follows shock annealing be effected as cold aging,essentially at room temperature.

An additional advantage of the present invention is achieved in that thesilicon precipitates are spheroidized in the eutectic phase and have across-sectional area A_(Si), of less than 4 μm².

The formula for determining the cross-sectional area is shown below:

$A_{Si} = {{\frac{1}{n}{\sum\limits_{k = 1}^{n}A}} \leq {4\mspace{11mu} {µm}^{2}}}$

wherein:

-   -   A_(Si)=average area of the silicon particles in μm²    -   A=average area of the silicon particles per image, in μm²    -   n=number of images sampled.

The advantages of a microstructure of this kind are essentially thatcrack initiation in the material is significantly reduced and ductilityof the material is improved by spheroidization of the Si precipitatesand by their fineness. In other words, the spheroidization and smallsize result in a favourable morphology of the brittle eutectic siliconand lead to significantly higher values for the material's elongation atfracture. In the case of mechanical loading, the stress peaks on theSi-Al phase boundary surface are reduced. A transcrystalline break wasalso found during tests, and this indicates the highest ductility of thematerial.

From the method standpoint, but also for high values of elongation atfracture, it may be advantageous if the silicon precipitates in theeutectic phase are spheroidized and have an average cross-sectional areaof less than 2 μm².

If, as was shown during development, the present invention entails thatthe average free path length between the silicon particles λ_(Si) in theeutectic phase defined as the root of a square measured surface dividedby the number of silicon particles contained within it is of a size thatis less than 4 μm, preferably less than 3 μm, and in particular lessthan 2 μm, an especially homogeneous stress distribution at low stresspeak values will be achieved in the material that is stressed, since thespacing between the small-area silicon particles significantly affectsthe flow behaviour of the material in a corresponding stress state.Determination of the distance between the silicon particles λ_(Si), isshown formally below.

$\lambda_{Si} = {{\frac{1}{n}{\sum\limits_{k = 1}^{n}\sqrt{\frac{A_{square}}{N_{Silicon}}}}} \leq {4\mspace{14mu} {µm}}}$

wherein

-   -   λ_(Si)=average spacing between of the silicon particles    -   A_(square)=square reference area in μm²    -   N_(Silicon)=number of silicon particles    -   n=number of images sampled.

Although solution annealing according to the prior art which is effectedas long-time annealing for 2 to 12 hours for a diffusion of the alloyingcomponents that are effective for hardening and the enrichment thereofin the mixed crystal entails spheroidization of the silicon particles asa secondary effect, the particles are very large and distributedunevenly as a result of the long annealing time; which can have adeleterious effect on the material's behaviour at fracture. It was mostsurprising that according to the present invention an eutectic siliconnetwork can be spheroidized by a shock annealing for a short time spanof just a few minutes, whereby an advantageous microstructure of thematerial can be achieved. In this connection, it is important that thetemperature for the shock annealing be as high as possible, althoughbelow the lowest smelting phase, preferably 5 to 20° C. below this.

With increasing annealing times, the silicon particles are subjected toa diffusion-controlled growth, and the initially favourable highspheroidization density ζ_(Si) becomes smaller.

In one aspect of the present invention, the highest ductility of anarticle of an Al—Si alloy was found if the mean spheroidization densityζ_(Si), defined as the number of spheroidized eutectic silicon particlesper 100 μm², has a value that is greater than 10, and preferably greaterthan 20.

$\xi_{Si} = {{\frac{1}{n}{\sum\limits_{k = 1}^{n}{\frac{N_{Si}}{A} \times 100}}} \geq 10}$

wherein

-   -   ζ_(Si)=average spheroidization density of the eutectic Si        particles    -   N_(Si)=number of silicon particles    -   A=reference area in μm²    -   n=number of images sampled.

The above is once again a formal specification of the formula.

Studies have shown that essentially each Al—Si alloy that contains theeutectic can be provided with a structure according to the presentinvention, and that the articles formed therefrom exhibit highmaterial-ductility values. The improvement of the quality and animprovement of elongation at fracture are particularly efficient if thearticle is manufactured by the thixocasting method.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in greater detail below on thebasis of test results and drawings appended hereto. The drawings showthe following:

FIG. 1: Bar chart showing mechanical values for a material as a functionof the thermal treatment state;

FIG. 2: As in FIG. 1

FIG. 3: SEM image of a cut

FIG. 4: As in FIG. 3

FIG. 5: Mean area of the Si precipitates as a function of the annealingtime

FIG. 6: As in FIG. 5

FIG. 7: Mean free path length between the Si particles

FIG. 8: Mean spheroidization density

FIG. 9: Bar chart showing material mechanical properties of variousAl—Si alloys

Table 1: Numerical values for FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, a bar chart shows the Rp_(0.2) limiting values and the valuesfor elongation at fracture A of samples manufactured from a testcomponent produced from an AlSi₇Mg_(0.3) alloy, said component havingbeen produced by the thixocasting method. The values for thermaltreatment state T6 (12 hours 540° C.+4 hours 160° C.) of the materialare compared to those that were achieved with the T6× method accordingto the present invention after shock annealing for 1 minute (T6×1),after 3 minutes (T6×3) and after 5 minutes (T6×5) at a temperature of540° C. All the samples were heat-aged (4 hours) at a temperature of160° C. The results of the tensile test show that the samples displaysignificantly higher values for elongation at fracture after shockannealing, the T6×3 effecting an increase of A by approximately 60% ascompared to T6.

In FIG. 2, using identically produced samples, the state values F, T4×3,T5, T6×3 and T6 are compared in a bar chart with respect to Rp_(0.2) andelongation at fracture A. When compared, they display marked increasesof the values for elongation at fracture. As can be seen from FIG. 2,the material can be cold-aged (T4×3) or heat-aged (T6×3) after shockannealing for 3 minutes in order to obtain superior elongation atfracture characteristics according to the present invention.

FIG. 3 and FIG. 4 show scanning electron microscope images of Siprecipitates. With respect to the imaging and evaluation method, it mustbe noted that it is essential to have binary images available in orderto permit quantitative evaluation. The images were taken with a scanningelectron microscope for an annealing period of 2 hours inclusive, afterwhich the cut was etched for 30 seconds using a solution of 99.5% waterand 0.5% hydrofluoric acid. After annealing for 4 hours, the cut wasetched with the Keller solution and the images could be taken by anoptical microscope. All the images were processed digitally using AdobePhotoshop 5, and evaluated with the Leica QWin V2.2 image analysissoftware; the minimal detection area amounted to 0.1 μm². FIG. 3 showsthe AlSi₇Mg_(0.3) after a normal T6 annealing time of 12 hours, using anSEM image. FIG. 4 shows the microstructure of the same material aftershock annealing for three minutes. It is clear that even after a veryshort time there is spheroidization of the silicon precipitates (FIG. 4)and the diffusion-controlled growth thereof after long annealing timescan be seen (FIG. 3).

FIG. 5 and FIG. 6 show the mean cross-sectional area A_(Si) of thesilicon particles during cut testing as a function of the annealing timeat 540° C. The increase of average cross-sectional area of the siliconparticles, which characterizes the size of the particles, can be clearlyseen from the details of FIG. 5 with the logarithmic time axis. Theincrease of the average silicon surface within the first 60 minutes,which is governed by diffusion, can be clearly seen from FIG. 6. Theaverage size of the silicon particles, which increases with annealingtime, is to a large extent dependent on the initial size of the siliconparticles in the eutectic. Since an extremely well refined and finelydivided silicon is present in this particular case, in some cases thatinvolve silicon particles that have not been refined so well, which isto say with initially larger silicon particles, the time within which acritical average silicon area A_(Si) of approximately 4 μm² can beachieved can become shorter.

The change of the average distance between the silicon particles as afunction of annealing time is shown in FIG. 7, using test results. Theincrease in the average spacing of the silicon inclusions can be clearlyseen.

Finally, FIG. 8 shows the decrease of the average spheroidizationdensity, ζ_(Si), as a function of annealing time. The sharp decrease ofthe average spheroidization density begins as soon as at 1.7 minutes anddrops to a pronounced loss of ductility starting at a value of <10 forζ_(Si). At higher annealing temperatures, this value may already bereached after 14 to 25 minutes, and a density value of greater than 20has to be provided for superior values of elongation at fracture.

The bar chart of FIG. 9 shows the measured values for yield strength andelongation at fracture which are listed in Table 1 for eight Al—Sialloys of different composition. In all of these alloys, an increase inthe ductility of the material is achieved according to the presentinvention.

TABLE 1 F T5 T6 × 3 T6 Variants Rp [MPa] A [%] Rp [MPa] A [%] Rp [MPa] A[%] Rp [MPa] A [%] AlSi7Mg03 121.7 13.0 167.5 9.9 228.5 16.7 259.8 10.6AlSi7Mg05 143.9 10.4 175.8 9.3 240.2 13.9 311.7 9.1 AlSi7Mgx 159.8 8.3197.2 6.8 265.2 10.1 322.9 7.6 AlSi6Mgx 159.7 10.2 195.3 7.8 250.6 8.9318.6 6.5 AlSi5Mgx 154.9 10.1 189.6 7.5 240.6 9.5 313.6 8.7 +Mn04 157.110.6 183.7 6.9 252.7 7.4 322.7 7.6 +Mn08 154.8 9.9 184.0 6.6 255.9 6.7324.4 4.9 AlSi5Mgxx 211.7 3.5 256.4 2.5 242.1 5.1 291.6 5.3

1. A thermal treatment process for improving the material ductility ofan article which comprises a cast or wrought aluminum-silicon alloy withan eutectic phase, which process comprises subjecting the article to anannealing treatment and a subsequent aging treatment, wherein theannealing treatment is carried out as a shock annealing treatment whichcomprises (a) a rapid heating of the material to an annealingtemperature of 400° C. to 555° C., (b) maintaining the material at thistemperature for a holding period of not more than 14.8 minutes, and (c)a subsequent forced cooling of the material to essentially roomtemperature.
 2. The process of claim 1, wherein the aluminum-siliconalloy further comprises one or more alloying elements.
 3. The process ofclaim 1, wherein the aluminum-silicon alloy further comprises one ormore contaminating elements.
 4. The process of claim 1, wherein thealuminum-silicon alloy further comprises at least one of Mg, Mn and Fe.5. The process of claim 1, wherein the aluminum-silicon alloy is atleast one of refined and purified.
 6. The process of claim 1, whereinthe holding period is shorter than 6.8 minutes.
 7. The process of claim1, wherein the holding period is not shorter than 1.7 minutes.
 8. Theprocess of claim 7, wherein the holding period is not longer than 5minutes.
 9. The process of claim 1, wherein the aging treatmentcomprises a treatment at a temperature of from 150° C. to 200° C. 10.The process of claim 9, wherein the aging treatment is carried out forfrom 1 to 14 hours.
 11. The process of claim 1, wherein the agingtreatment comprises a cold aging treatment at essentially roomtemperature.
 12. The process of claim 1, wherein the holding period isfrom 1.7 to less than 6.8 minutes and the aging treatment comprises atreatment at a temperature of from 150° C. to 200° for from 1 to 14hours.
 13. The process of claim 1, wherein the holding period is from1.7 to less than 6.8 minutes and the aging treatment comprises a coldaging treatment at essentially room temperature.
 14. The process ifclaim 1, wherein the article is made by a thixocasting method.
 15. Theprocess if claim 1, wherein the entire article is subjected to theannealing treatment.
 16. The process if claim 1, wherein the eutecticphase consists essentially of an α_(Al)-matrix and spheroidized siliconprecipitates, which precipitates have an average cross-sectional area,A_(Si), of not more than 4 μm², A_(Si) being represented by thefollowing equation:$A_{Si} = {{\frac{1}{n}{\sum\limits_{k = 1}^{n}A}} \leq {4\mspace{11mu} {µm}^{2}}}$wherein A_(Si)=average area of the silicon particles in μm² A=averagearea of the silicon particles per image, in μm² n=number of imagessampled.
 17. The process of claim 1, wherein the eutectic phase consistsessentially of an α_(Al)-matrix and silicon precipitates, the siliconprecipitates comprising silicon particles and an average free pathlength between the silicon particles, λ_(Si), in the eutectic phasebeing not higher than 4 μm, λ_(Si) being represented by the followingequation:$\lambda_{Si} = {{\frac{1}{n}{\sum\limits_{k = 1}^{n}\sqrt{\frac{A_{square}}{N_{Silicon}}}}} \leq {4\mspace{11mu} {µm}}}$wherein λ_(Si)=average spacing between the silicon particlesA_(square)=square reference area in μm² N_(Silicon)=number of siliconparticles n=number of images sampled.
 18. The process of claim 1,wherein the the eutectic phase consists essentially of an α_(Al)-matrixand silicon precipitates, which silicon precipitates comprise siliconparticles and an average spheroidization density, ζ_(Si), thereof,defined as a number of silicon particles per 100 μm², is at least 10:$\xi_{Si} = {{\frac{1}{n}{\sum\limits_{k = 1}^{n}{\frac{N_{Si}}{A} \times 100}}} \geq 10}$wherein ζ_(Si)=average spheroidization density of the eutectic Siparticles N_(Si)=number of silicon particles A=reference area in μm²n=number of images sampled.
 19. A thermal treatment process forimproving the material ductility of an article which comprises a cast orwrought aluminum-silicon alloy with an eutectic phase, which processcomprises subjecting the entire article to an annealing treatment and asubsequent aging treatment, wherein the annealing treatment is carriedout as a shock annealing treatment which comprises (a) a rapid heatingof the material to an annealing temperature of 400° C. to 555° C., (b)maintaining the material at this temperature for a holding period of notmore than 5.8 minutes, and (c) a subsequent forced cooling of thematerial to essentially room temperature.
 20. The process of claim 19,wherein the holding period is not longer than 5 minutes.